The present invention relates to optical lithography, and more particularly to various methods of fabricating alternating phase shift masks that can be used to pattern various semiconducting devices via an optical lithographic process.
Phase shifting technology is one of the more useful methods available to enhance the performance of optical lithography. A phase shifting mask normally consists of an absorbing phase shifting material having from about 5 to about 20% transmittance and a transparent or semi-transparent substrate such as quartz.
Currently, the process of creating typical alternating phase shift masks intentionally creates a step feature of up to about 3000 xc3x85 in the transparent or semi-transparent substrate. Such a prior art process is shown, for example, in FIGS. 1A-1D. Specifically, FIG. 1A shows an initial structure that is typically employed in the prior art in fabricating an alternating phase shift mask. The initial structure shown in FIG. 1A comprises transparent or semi-transparent substrate 10, phase shift material 12 that is formed on a portion of substrate 10, and a patterned resist 14 which is formed atop a portion of phase shift material 12 as well as portion of substrate 10. The initial structure shown is fabricated utilizing conventional processes well known in the art; therefore a detailed discussion concerning the same is not needed herein.
Next, and as shown in FIG. 1B, the initial structure is then subjected to a conventional dry etching process such as reactive-ion etching (RIE) which is capable of selectively removing the exposed portions of substrate 10. The resultant structure shown in FIG. 1B is then subjected to a conventional etch back process which results in the formation of the structure shown, for example, in FIG. 1C. The patterned resist is then removed utilizing a conventional stripping process so as to provide the mask shown in FIG. 1D. Due to the processing steps described above, step region 16 is formed in a portion of the substrate which is opposite to that of phase shift material 12. Step region 16 is undesirable since it results in a printable defect region in the mask.
There is an artifact of the prior art etching process that creates this step region where a phase shift is not required. The xe2x80x9cextraxe2x80x9d step in the substrate will create printable lines in the semiconductor wafers due to the effects of the phase shift of light and the sensitivity of the resist on the semiconductor wafer.
One solution to the above problem for the device lines is to create a second mask that can be exposed on the same wafer to eliminate this region of printable lines. This type of mask is known in the art as a xe2x80x9ctrimxe2x80x9d mask. Although capable of removing the unwanted printable lines caused by the steps in the substrate, the use of trim masks requires a two-step process in the device lines for every wafer.
In view of the above problem with fabricating prior art phase shift masks, there is a continued need for developing a new and improved method of fabricating alternating phase shift masks that eliminates the need of using a trim mask to remove the unwanted printable lines.
One object of the present invention is to provide a method of fabricating an alternating phase shift mask wherein unwanted printable lines are not printed on a semiconductor wafer during patterning via an optical lithographic process.
A further object of the present invention is to provide a method of fabricating an alternating phase shift mask wherein the use of trim masks to remove unwanted printable lines can be eliminated.
A yet further object of the present invention is to provide a method of fabricating an alternating phase shift mask which employs relatively simple processing steps that eliminate the formation of step regions within the transparent or semi-transparent substrate.
These and other objects and advantages are achieved in the present invention by adding additional processing steps to the phase shift mask manufacturing process so to eliminate the edges of the step regions that cause printing of the undesirable printable lines.
In a first method of the present invention, the elimination of the edges of the step regions can be obtained in the present invention by utilizing the following processing steps which include the steps of:
(a1) forming at least one opaque image on a surface of a substrate;
(b1), forming a hardened, patterned first resist on portions of said substrate including said at least one opaque image, while leaving other portions of said substrate exposed;
(c1) forming a patterned second resist so that at least one edge of said second resist is located in areas susceptible to printable step defects and reflowing said second resist to obtain a sloped profile at said at least one edge;
(d1) transferring said sloped profile of said patterned second resist to said substrate, while maintaining a substantially vertical profile in said substrate beneath said at least one opaque image; and
(e1) removing said first and second resists.
Several different embodiments can be used in the first method of the present invention in transferring the sloped profile of the patterned second resist to the substrate. In one embodiment of the present invention, the sloped resist profile is transferred to the substrate utilizing a dry etching process such as plasma etching or laser ablation. In another embodiment of the present invention, the sloped resist profile is transferred to the substrate utilizing a combination of dry etching and ashing. In yet another embodiment of the present invention, step (d1) comprises ion implanting through the second resist into the substrate so as to create an implant profile in the substrate and selectively removing said implant profile utilizing a wet etch process or a laser ablation process such as a femtosecond laser ablation process.
It is noted that the present invention also works in cases wherein the second resist denoted in step (c1) is not utilized. In such an embodiment, a localized implant using a light-absorbing ion such as Ga is employed in creating the sloped profile in the substrate. The sloped profile created in the substrate is then removed utilizing a wet etch process or a laser ablation process such as a femtosecond laser ablation process.
In a second method of the present invention, the elimination of the edges of the step regions can be obtained in the present invention by utilizing the following method which includes the steps of:
(a2) forming at least one opaque image on a surface of a substrate;
(b2) forming a hardened, patterned first resist on portions of said substrate including said at least one opaque image, while leaving other portions of said substrate exposed;
(c2) forming a patterned second resist so that at least one edge of said second resist is located in areas susceptible to printable step defects and implanting a sloped profile into said substrate;
(d2) removing said sloped profile from said substrate, while maintaining a substantially vertical profile in said substrate beneath said at least one opaque image; and
(e2) removing said first and second resists.
It is noted that step (d2) of the second method of the present invention, may include a chemical removal process or a laser removal process such as a femtosecond laser ablation process. In the second method of the present invention, it is also possible to omit the use of the second resist in step (c2) and to create the sloped profile in the substrate by utilizing a controlled ion implant process wherein light absorbing ions such as Ga are employed.